SAO PAULO, Oct 23 (Reuters) - Brazil's leading presidential hopeful, far-right candidate Jair Bolsonaro, said on Tuesday that he will not debate his leftist rival before Sunday's election, and sought to counter criticism that he represents a threat to the country's democracy.

Bolsonaro, who was stabbed last month while campaigning, said in a radio interview that he was not physically fit enough to attend any debate.

His rival for the presidency, Fernando Haddad of the Workers Party (PT), has demanded that he and Bolsonaro face off anywhere and in any format, saying Brazil's democracy will suffer if voters cannot hear and compare their competing ideas.

Haddad and his allies have said they are gravely concerned that Bolsonaro, a 63-year-old retired Army captain and seven-term congressman who has openly praised Brazil's 1964-1985 military dictatorship, could fall back into authoritarian ways.

"We are not a threat to democracy. On the contrary, we are the guarantee for liberty and democracy," said Bolsonaro, who opinion polls show has a lead of around 18 percentage points over Haddad ahead of Sunday's run-off vote.

Speaking to a local radio station, Bolsonaro pointed to the political corruption that grew under successive PT governments and involved every major party as evidence that a Haddad government would allow graft to continue. Haddad has said he would fight against corruption.

While his defense of the military government and offensive comments about minorities have angered critics, Bolsonaro has not been hit by corruption charges – a selling point with voters weary of economic crisis and graft scandals.

Bolsonaro accused the PT of "firing blanks" with its petition before the country's top electoral court that it investigate his campaign's for allegedly violating campaign law with a misinformation campaign on Facebook Inc's WhatsApp messaging service.

Bolsonaro denies he has any connection to coordinated attacks via WhatsApp, and has said that he cannot control what his backers may send over the platform. (Reporting by Brad Brooks in Sao Paulo and Pedro Fonseca in Rio de Janeiro Editing by Frances Kerry)